It's a dramatic countdown: you have just two weeks to protect yourself against a "powerful computer attack", the National Crime Agency has warned.

All very ominous, but how serious is the threat?

Around 15,500 computers are already infected in the UK. To put that in context, there are at least 21 million computers in the country, before secondary devices like laptops, tablets and phones are counted. That's less than a 0.1% infection rate.

And although Game Over Zeus - one of the pieces of malware involved - has caused some serious financial harm to individuals, authorities estimate it is responsible for hundreds of millions of pound of fraud.

To put that in context, cybercrime worldwide costs around £60bn annually, according to one estimate.

So why the stark warning?

First, the botnet, the network of infected computers controlled by criminals, is pretty resilient.

It works on a peer-to-peer mechanism, and uses a lot of encryption. Compared to other botnets, which can sprawl into the millions, this one was tightly controlled, by gangs from Russia and Ukraine.

That's who the NCA, FBI and US Department of Justice have moved against, and that's unusual.

The authorities rarely seize or disrupt a botnet. They can do so for only two weeks, as it's an expensive operation.

At the same time, they're aiming to arrest the gang's ringleaders through more traditional, legal means.

But it's a new line in the sand, showing that international forces are willing and able to collaborate on cybercrime.

Pity, then, that the UK government's Get Safe Online site wasn't quite so joined up: it crashed under the weight of concerned internet users.